Background: Growing evidence now links the built environment to physical activity, dietary quality, and obesity. The goal of this study is to assess the influence of the built environment on longitudinal changes in body mass index (BMI) in a cohort of 11,797 children from 16 communities across Southern California. This study will focus on the contributing role of neighborhood-level factors to the progression toward overweight and obesity or "obesogenic trajectories". We define these trajectories as the temporal progression toward overweight and obesity compared to age-adjusted growth curves for the cohort. Methods: We build on over 8 yrs of measurements on 6,259 children in the DSC Children's Health Study (CHS) (ages 9-10 at enrollment, reaching 18 years at end of follow up). We will supplement this data set with 4+ years of follow up on 5,538 children in a new cohort (ages 6-7 years at recruitment with follow up until they are 11-12 years). Participants have been thoroughly characterized with annual measurements of height and weight, lung function, physical activity, baseline dietary intake, gender, race, ethnicity, and socioeconomic status (SES). CHS data will be integrated with measures of the built environment derived in a Geographic Information System. We will test the impact of the built environment and obesogenic trajectories using spatial statistics and multilevel growth curve models. Specific Aims: (1) To assess the effects of the neighborhood built environment on obesogenic trajectories and (2) To explore whether individual (i.e., gender, race, SES) and contextual variables (i.e., air pollution) modify the association between the built environment and obesogenic trajectories. Significance: Specific strengths of this application include the examination of the effects of the built environment on children's prospective change in weight status, direct assessment of children's weight status annually, efficient use of existing environmental and individual data, and the ability to evaluate potential differential effects across ethnicity/race on the relation between built environment and obesity. This project will identify specific variables in the built environment that significantly influence the development of obesity in children. These findings could have public health implications with respect to structuring the built environment to prevent obesity in children.